I also had Thetargos's problem after upgrading from F10 to F11, which upgraded to TB 3.0b2. As for Imgutie, I had no master password, but TB thought I had one and would not allow me to disable it by unchecking "Use a master password" in Edit -> Preferences -> Privacy -> Passwords.

Imgutie's solution (disable the master password in Firefox) didn't work for me, but the solution I eventually found is obviously very nearly the same. I found that the instructions for resetting the master password in TB 2 at http://kb.mozillazine.org/Master_password were the key:

"Choose Tools – Error Console, paste the expression:
openDialog("chrome://pippki/content/resetpassword.xul")
and press the Evaluate button. That will open a dialog asking you if you want to reset your password."

After confirming that I wanted to reset my master password, TB popped up a dialog that allowed me to set a new master password without requiring entry of the current (non-existent) master password. I did so, then I returned to Edit -> Preferences -> Privacy -> Passwords and unchecked "Use a master password". As before, TB asked for the master password (which I had just set); I entered it, and <presto!> the problem was solved.

Having said all this, I recognize that it's probably a good idea to have and use a master password, but it is not trivial to discover how to set one after an upgrade. I can only imagine that upgrading to F11 will cost TB many users who will abandon it in frustration as Thetargos apparently did, after discovering that it has become unusable. I hope this note helps anyone who hasn't given up yet!